Only real difference is the upper receiver. The Entry Tactical and Tactcal CAR A4 use an A4 upper receiver, which is probably the most versatile platform for optic and accessory mounting options. The UTE 2 provides some of that versatility withthe A$ height rail that runs about two-thirds of its length, in front of the fixed A2 rear sight. If you're going to run red dot sights and other accessories, it will work just fine. If you're looking at going to a magnifying scope witha limited eye relief, you would have to mount it quite hight to get over the rear sight.
Advantages and disadvantages are goign to vary from shooter to shooter, and even for the same shooter with different desired uses for a rifle.
Sorry...I missed the part about the Elite. The Elite does use a mid-length gas system, which gives you 2" longer gas system, sight radius, and handguards. Also a lighter gas strike onthe carrier, lower felt recoil impulse on the buffer, and should prolong the life of your gas rings.